1. Field of the Invention
As described in U.S. Pat. Nos. 3,583,887 and 3,841,925, silicon-containing steel sheet is prepared by cold-rolling silicon-containing steel into sheet form, coiling the steel sheet into rolls and thereafter annealing the coiled sheet by a controlled heating process to produce a grain oriented steel having desirable magnetic properties.
Magnesium oxide is used extensively as a highly heat resistant separator medium and protective coating for metal surfaces. It is also used as an electrical insulator coating for metals, as a "gatherer" for removing impurities, such as sulfur and carbon, from thin metal sheets and particularly as a protective coating for silicon steel. The electrical insulating coating is understood to be derived from coating magnesium oxide on steel, which then forms a film or coating containing Mg.sub.2 SiO.sub.4 which coating is an effective electrical insulator when annealed.
2. Description of the Prior Art
Referring again to U.S. Pat. Nos. 3,583,887 and 3,841,925, according to the present industrial practice, silicon-containing steel is cold rolled into sheets, decarburized and thereafter coiled into convenient rolls. Cold rolling develops in the steel the potential to form a grain oriented structure when the steel is later "annealed." The term annealed refers to a process whereby the steel is heated to about 1200.degree.C. in an atmosphere of low dewpoint containing hydrogen, or in a vacuum, under programmed conditions with respect to time and temperature. This results in a growth in size of the steel grains and also in a specific grain orientation which provides the desired soft magnetic properties sought. During the annealing process, virtually all of the remaining excess carbon and sulfur content of the steel is lost.
During the annealing process in hydrogen where the steel is in large coils, in the absence of a suitable separating medium, the coiled roll would fuse to itself and could not be unrolled. Conventionally, this is avoided by placing a thin coating of magnesium oxide on the steel prior to coiling. Further, the magnesium oxide coating serves to reduce impurities such as carbon and sulfur in steel by chemical reaction. In addition, magnesium oxide provides a major part of an electrically insulating silicate layer by reaction with the steel. For most applications, this silicate insulation is important to form an efficient electrical insulating coating or as the base coating upon which such insulation is formed. Thus, for example, transformer cores are constructed from thin sheets of soft magnetic steel stacked together to form a laminated body in which each sheet is electrically insulated from its neighbor. This construction vastly reduces the generation of eddy currents in the core imposed by an alternating electrical field. The average density of soft iron in the core should be as large as possible and consequently the insulation on the plates should be as thin as possible to provide closer stacking of steel plates.
Obviously, short circuits between plates reduces transformer efficiency and often causes the development of damaging hot spots in the transformer core. Consequently, soft magnetic steel is rated by the number of short circuits per unit area, usually expressed in terms of the electrical resistance of the insulating layer. This is a standard ASTM measurement known as the Franklin Test and is a measure of the conductance.
Generally, it is known that two types of oxidizing agents added to MgO aid in forming high quality MgO insulation on silicon steel, i.e., high and low temperature oxidizers. High temperature oxidation supplies oxygen to reactions that can be summarized as: EQU O.sub.2 + Si + 2 MgO .sup.High temp. Mg.sub.2 SiO.sub.4 ( 1)
Low temperature oxidation is for reactions of the following form: EQU 3 Fe + 2 O.sub.2 .sup.Lower temp. Fe.sub.3 O.sub.4 ( 2)
It appears that Fe.sub.3 O.sub.4 and associated iron silicates serve as a bonding layer on steel surfaces which holds the Mg.sub.2 SiO.sub.4 insulating layer to the steel. The iron oxide might also supply some of the oxygen for reaction (1).
By high temperatures is meant those temperatures reached when Mg.sub.2 SiO.sub.4 forms from MgO and silicon steel during hydrogen annealing. By lower temperatures is meant temperatures at and below those reached on the steel when drying wet coatings.
Reaction (2) commonly occurs by reaction of steam and iron, i.e., EQU 4 H.sub.2 O + 3 Fe .fwdarw. Fe.sub.3 O.sub.4 + H.sub.2 .uparw.(3)
and somewhat by EQU 2 O.sub.2 + 3 Fe .fwdarw. Fe.sub.3 O.sub.4 ( 4)
These reactions occur in the last stages of decarburizing the steel and more commonly when heat drying wet coatings on the steel. In either case the oxide layer is visible in the form of a temper color of the immediate steel surface if the layer is thick enough to cause light diffraction.
Reactions of the type described in equation (1) have been taught for many years and many high temperature oxidizing agents are known. Among these are manganese, zinc, silicon, and chromium oxides. Manganese is particularly preferred since it can preferentially oxidize silicon and not iron when present in the reduced state. As with low temperature oxidizers, problems are encountered when attempting to coat and distribute high temperature oxidizers in the coatings. Hence, appreciable quantities of manganese, zinc, or silicon oxides cannot be introduced without destroying the needed rheological properties of the coating slurry.
The patent literature teaches the following regarding the addition of Zn and Mn to MgO:
1. Manganese oxides used in conjunction with Al.sub.2 O.sub.3 aid in forming a uniform glass (see U.S. Pat. Nos. 3,627,594 and 3,522,108).
2. MnO plus MgO is used to improve the adhesion of the base insulation to the steel. Zn, Cu, or Cr oxides can also be substituted for Mn (see U.S. Pat. No. 3,522,108).
3. MnO can selectively oxidize Si at 800.degree.C. without oxidizing Fe during hydrogen annealing because of the free energy relationship that exists. MnO differs from Cr.sub.2 O.sub.3 in that it is entirely assimilated by the MgO and the steel. However MnO is not commercially available and must be prepared by reduction of MnO.sub.2 (see Ger. Pat. Nos. 2,033,650 and 2,062,290). ZnO was also advocated for making insulation coatings in U.S. Pat. Nos. 3,364,057 and 3,271,718. MgO plus reduced manganese oxide can tolerate up to 3% MnO.sub.2 or 25% Mn.sub.3 O.sub.4. The MgO can be dead burned.
It is also known that MnO.sub.2 admixed with MgO produces poorly adhering coatings and that MnO.sub.2 tends to separate out of suspension.
It has been discovered that the condition of the Fe.sub.3 O.sub.4 on the steel affects the quality of the insulation eventually formed after annealing. Apparently an optimum thickness of a highly uniform layer is required after drying coatings and before annealing. The source of oxygen for reactions of the type (3) and (4) and the rate at which reaction occurs can apparently determine what type of oxide layer forms. I have accordingly discovered that certain well known oxidizing agents when incorporated with magnesia in aqueous coating slurries will provide the type of oxide layers sought. However, because of rheological alterations that occur when such agents are added to magnesia slurries the choice is limited and none have been found that will perform properly without incorporating a second additive which modifies the coating behavior.
It has been discovered that the elements represented in the oxides of zinc and manganese can be introduced as a single chemical compound as zinc permanganate. I have further discovered that by introducing this water soluble compound in combination with a sulfate promoter that I can improve both forms of oxidative functions described by equations (1) and (2) and at the same time coat the steel successfully.
It has also been discovered that when said additives in combination with magnesia are coated on high permeability steels which contain aluminum we obtain superb insulation. X-ray studies revealed that the zinc can combine with aluminum and oxygen to provide zinc aluminate which in combination with the magnesium silicate formed provides good insulation.
Zinc permanganate admixed with magnesia and water tends to hydrolize forming zinc and manganese hydroxides. This formation tends to destroy the rheology of the coating suspension and defeats the object of introducing a water soluble additive. Accordingly, I found that the hydrolysis could be suppressed by adding a water soluble alkali earth metal sulfate. MgSO.sub.4 is preferable since it does not introduce new elemental species into the formula.
Accordingly, it would be desirable to provide magnesium oxide coating compositions which have high and low temperature oxidizing properties and which when coated onto silicon steel and thereafter annealed produce a high quality electrical insulation.
It is therefore an object of the present invention to provide magnesium oxide silicon-steel coating compositions which have high and low temperature oxidizing properties and which provide high quality electrical insulation when coated onto steel and thereafter annealed. It is another object to provide said coating compositions in a form which exhibit good rheological properties when formed into an aqueous slurry for use in a steel coating operation.